A flat, if untidy, can be deadly (5)

I believe the answer is:

fatal

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'be deadly' is the definition.
(similar in meaning)

'a flat if untidy' is the wordplay.
'a flat' anagrams to 'FATAL'.
However, I'm unsure how this is indicated.

'can' acts as a link.

Can you help me to learn more?

(Other definitions for fatal that I've seen before include "Calamitous" , "Bringing death" , "making one late" , "Unavoidable" , "critical" .)

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